The invention relates to a process for the melting, refining and homogenizing of glass, There, raw materials such as silicates and a gas batch are first melted up in a tank or in a crucible. In the melting the Skull principle is being used to an increasing degree. Here, high-frequency energy is sent into the content of the crucible by means of an induction coil. The glass melt is then transferred into a refining vessel. Here too, the Skull principle is again applied. Finally, the purified glass melt passes into a homogenizing vessel. Reference is made of WO 98/18731 and to WO 98/03442.
In the homogenizing and conditioning of glass melts, platinum structural components are used. These have, to be sure, the advantage of a high resistance to corrosion. It is known, however, that oxygen bubbles occur as soon as the melt comes in contact with platinum parts. As is well known, platinum has a catalytic decomposition effect on water. Platinum is permeable to hydrogen, so that there may occur a hydrogen diffusion through the platinum. If the hydrogen content differs between the outside and the inside of the wall of a platinum structural component, then there occurs a steady hydrogen transport in one and the same direction. Since the partial pressure of the water in a glass melt is higher then the environmental partial pressure, there occurs a depositing of oxygen on the inner wall of the platinum structural component. When the solubility limit for oxygen in the melt is exceeded, a bubble formation occurs, the so-called oxygen reboil on platinum.
Attempts have already been made to suppress the reboil mentioned by countermeasures. Thus, on the outside wall of a platinum vessel a controlled hydrogen atmosphere has been provided. It has also been attempted to establish corresponding conditions in the melt itself, for example, to adjust the oxygen content of the melt in a defined manner and simultaneously to expose the melt to an oxy-fuel heating. These measures, however, require expensive apparatuses and are correspondingly costly. The problem of adjusting the water content has, furthermore, the disadvantage that with a change in the water content the product properties are altered, which is undesired.
Underlying the invention is the problem of designing a process for the melting, refining and homogenizing of glass, in such manner that also with use of platinum structural components the oxygen reboil mentioned is avoided.
The problem is solved by the features of claim 1.
The inventors have perceived the following:
The oxygen reboil tendency of a glass melt decreases if the temperature of the melt is raised to a certain minimum level on the way to the homogenizing station and if, furthermore, polyvalent ions are present in the melt. The ions can be present, for example, in the form of vanadium, cerium, zinc, tin, titanium, iron, molybdenum, or europium. The temperature of the melt should be higher than 1700xc2x0 C., and better still, higher than 2400xc2x0 C.
The inventors have perceived in detail the following: the ions are reduced at the high melting or refining temperatures mentioned. Thus, for example V5+ at temperatures around 2200xc2x0 C. goes over into V3+. Ti3+ becomes Ti2+. It is not disadvantageous for the temperature to be lower in a homogenizing or conditioning process. Here the higher valency state of the polyvalent ions remains stable. In order to reach the higher valency state, the ion needs oxygen which normally is not present in fully refined melt. If a water decomposition occurs when the melt comes in contact with a platinum structural component, to be sure, oxygen is generated. This, however, does not lead to reboil bubble formation. On the contrary, the oxygen is buffered off from the polyvalent ions present in the reduced state.
A great advantage of the inventive process of the invention lies in that the supplying of toxic refining agents such as arsenic oxide or antimony oxide is not necessary. This on the one hand lowers the costs and, on the other hand, reduces the known risks.
In one form thereof, the present invention provides a process for producing a glass melt, including the steps of melting glass in a first stage; refining the melt in a second stage, the melt having a polyvalent ion content of at least 0.5 wt. %, with at least one of said melting and refining steps conducted at a temperature of at least 1800xc2x0 C.; and homogenizing and conditioning the glass in a third stage. In a further form, at least one of said melting and refining steps is conducted at a temperature of between 2100xc2x0 C. and 2400xc2x0 C. and above.